A blog for the cryptography group of the University of Bristol. To enable discussion on cryptography and other matters related to our research.

Monday, March 4, 2013

Crypto is Dead; Long Live Crypto!

At last week's RSA conference, in the Cryptographer's Panel, Adi Shamir said that we need to prepare for a post-crypto world. The same basic idea was put forward by Jonathan Zittrain in his invited talk at last years CRYPTO conference, which was entitled "The End of Crypto". The basic idea behind both statements is that in the modern world the attacker is already inside the box, so no matter how much encryption you use the bad guy will always get your data.

Let us elaborate on this idea a bit more. In the consumer field you may use your smart phone to store all sorts of important data on it (email addresses, passwords, banking data etc), but most users also download apps. Very few of these apps have had any form of code or security review, and so could contain malicious code which can compromise your phone, and your security.

In the enterprise space the problem is even more acute. The existance of APTs (Advanced Persistant Threats) means that enterprises need to assume that the attacker is already within their perimeter. This has always been true, in that often attacks come from the inside, but now we have seen external attacks which are based on dorment code, lying in wait, within an enterprises security perimeter.

The usual cryptographic model is that there is at least one honest person in a protocol. This is still true, the user of a system may be honest, but if we cannot trust the system they are on to not be compromised, we can essentially not trust anything. What is the point of encrypting data if the attacker already has the key? Even if the data has been encrypted successfully, at some point it needs to be decrypted so as to process it. At the point of decryption, if the attacker controls the machine, he also controls the decrypted data. So with such all pervasive threats, how can crypto be of any use what-so-ever? In some sense this means "Crypto is Dead!".

Luckily, however, cryptography is about more than just encryption and authentication. Modern cryptography gives us a number of tools which we can use to secure systems. In Bristol we have been working for a number of years in turning a theoretical technology called multi-party computation (MPC) into a practical reality. The basic idea behind MPC is the following: Suppose a set of n parties have their own secret inputs, for example party i has input xi. Now suppose they want to compute some function on these inputs say

f(x1,...,xn)

MPC allows us to do this, via a protocol, in such a way that the parties learn the output of the function, but no one learns anything about the other parties inputs. Traditional use-cases which have been proposed for MPC are the sharing of databases etc.

However, MPC also allows one to solve the problem of what hardware to trust in an enterprise environment such as that described above. In other words we can use MPC as a threat mitigation strategy to avoid APTs and other malware on our systems. We do this by altering the standard use case of MPC, instead of having multiple parties we have one party who just happens to be using multiple computers.

Suppose we expect that an attacker is inside our network. If we put in place enough protection to ensure he is not on all of our computers at once, then we may be able to hold a secret key on that uncompromised computer. But we do not know which computer is uncompromised. Using MPC we can get around this. Consider the simplified situation where we have two computers, where we know it is likely one is compromised and the other is not. We now "split" the encryption key between the two servers, for example via a one-time pad. So each party essentially has a random string, but the two parties together when combining their random string have the secret key. We treat the two random strings as the inputs x1 and x2 above, and now consider f as the encryption function. In this way the two servers can perform the encryption, without anybody ever knowing the underlying secret key itself.

We could use the same idea for messages. Instead of having one server needing to decrypt some ciphertext, with the obvious problem of what happens when the server is compromised, a combination of servers could use MPC to decrypt the ciphertext and then use MPC to process the plaintext. In this way no single party ever sees the underlying key, and the underlying plaintexts, at all.

This might seem like pie in the sky, but we can actually perform such computations today, using protocols developed in Bristol and at other places around the world. For example when we first implemented the AES functionality in a covertly secure manner back in 2009 we could evaluate a single AES block encryption between two partes in 60 seconds. Now this can take as little at 0.012 seconds, and we can process around 1000 such encryptions per second. With these latencies and throughputs we can actually implement solutions which help mitigate against the problem of servers being compromised by APTs.

Of course MPC only acts as a threat mitigation defence in such situations. If all machines are compromised, then all bets are off. But that is the same as with all security mechanisms, one needs to combine them together to provide a secure system. But the key message is that with MPC one no longer needs to place ones secrets all in one place. So whilst the traditional cryptographic model may be dead in the modern world, the application of cryptography to address security problems is far from dead. So Long Live Crypto!